I. Field of the Invention
The invention is directed to VOC (volatile organic component)-containing flock adhesive compositions based on isocyanate terminated polyols. More particularly, the invention is directed to flock adhesive compositions having reduced amounts of VOC, yet having low viscosity, high percent solids, and long-term adhesion to elastomer substrates over a wide temperature range.
II. Description of the Related Art
The preparation of flocked rubber articles wherein flocks are fixed to the surface of the rubber material through an adhesive layer such as a polyurethane are well known. In such processes, the rubber surface is coated with a layer of the adhesive and short staple fibers are applied perpendicularly thereof utilizing an electrical field. The resulting electrostatically flocked articles are characterized by a fabric-like surface of relatively low friction. This feature renders flocked rubber articles particularly suitable for the purpose of reducing friction between the sliding glass window and window channel of automobiles.
In U.S. Pat. No. 6,262,217, there are taught polyurethane compositions containing a polyurethane prepolymer component of an isocyanate-terminated polyalkadiene having a molecular weight from about 1500 to about 5000 grams per mol (g/mol) and an additional isocyanate-terminated polyalkadiene with a molecular weight from about 500 to about 1500 g/mol.
The curable liquid materials of choice are isocyanate-terminated polyurethane prepolymers. Typical formulations contain 40% by weight polyurethane prepolymer and 60% by weight of a readily volatile (VOC) solvent as a diluent, needed to control viscosity and coatability. Dilution in the solvent gives a sufficiently low application viscosity so that the adhesive can be applied by drip and drag techniques, casting, by roller or by spraying. The most common VOC solvents used include xylene, toluene, ketone mixtures or chlorinated hydrocarbons. In some fiber flock processes much of the solvent is volatilized prior to flocking by high-voltage electrical fields in a flocking cubicle. Ultimately, solvents are emitted in the process and environmental remediation is needed in accordance with environmental standards.
It is known to provide flock adhesive compositions based upon isocyanate prepolymers, in combination with dinitrosobenzene and epoxy-novolaks. Typical examples of such solvent-containing flocking adhesives are disclosed in U.S. Pat. No. 4,535,121. These adhesives contain polyurethane prepolymers based on diisocyanates and polyols, aromatic dinitroso compounds as adhesion promoters, polyfunctional reaction products of isocyanates and epoxide compounds, low-boiling solvents such as toluene or xylene, as well as conventional fillers, pigments and/or stabilizers. The reaction products of isocyanates and epoxide compounds are produced in the absence of water using reaction catalysts which bring about a direct reaction between the epoxide groups and the isocyanate groups. Examples of such catalysts are alkali phenolates, acetates, lactates, naphthenates and alcoholates, and also alkali salts of fatty acids such as lithium stearate. These reaction products contain no epoxide groups.
U.S. Pat. No 4,535,121 discloses an adhesive for flocking elastomers, more especially apolar elastomers, such as EPDM. The adhesive contains an isocyanate-terminated polyurethane prepolymer and, as an adhesion promoter, a reaction product of an aromatic diisocyanate with a polyfunctional epoxide, particularly triglycidyl isocyanurate. The strength of adhesion and resistance of the bonds can be further increased by the addition of aromatic nitroso compounds, for example, 1,4-dinitrosobenzene. The isocyanate terminated urethane prepolymers suggested in U.S. '121 are polypropylene glycols having a molecular weight of from about 500 to about 5000 and, preferably, from about 1000 to about 2000. Other isocyanate-terminated prepolymers suggested are reaction products of castor oil with diphenylmethane diisocyanate or other aromatic diisocyanates. Reaction products of polybutadiene polyols are also suggested. Particularly suitable polybutadiene polyols have molecular weights of from about 500 to about 6000 and more particularly from about 2000 to about 4000 with an OH number of, for example, from about 30 to about 80. Experience has shown that flock adhesives made with isocyanate prepolymers based upon a polyol having an average functionality of 2 for bonding fibers to underlying elastomer substrates do not exhibit acceptable resistance to deformation at high temperatures, such as in a heated molding process used to join two flocked substrates together, e.g., as in a corner.
U.S. Pat. No. 4,835,226 discloses a flock adhesive composition comprising 1 to 20% by weight of an aromatic nitroso compound; about 1 to 20% by weight of an epoxy resin having an epoxy equivalent of at least one; and 60 to 98% by weight of a blocked isocyanate-functional urethane prepolymer. The isocyanate groups of the prepolymer are blocked with a substituted or unsubstituted phenol. The urethane prepolymer has a molecular weight of at least 500, particularly in the range of 1,000 to 10,000, being preferred. In such flock adhesives the residual monomeric isocyanate present will also be converted to the blocked form. Conventional isocyanate prepolymers contain residual monomer—typically in excess of 0.5 wt. % and up to several percent. Due to volatility, monomeric isocyanates are more hazardous than bound or terminal isocyanates and are the major reason why isocyanate prepolymers are blocked. Experience has shown that by blocking free and bound isocyanates on prepolymers and in particular, blocking the isocyanates on the prepolymer backbone, leads to slower cure kinetics and a viscosity increase. The viscosity increase must be compensated with higher solvent diluent levels to render these adhesive fluids adaptable for coating elastomer substrates. It would be desirable to minimize viscosity, reduce VOC content and provide flock adhesives that are essentially free of volatile monomeric isocyanates (<0.1% free isocyanate).
U.S. Pat. No. 4,918,119 discloses a moisture-hardening flocking adhesive for elastomer flocking containing a C8-C16 substituted diluent which is not a volatile organic component and is not driven off prior to electrostatic application of flock fibers. The adhesive contains an isocyanate terminated polyurethane prepolymer, an aromatic dinitroso compound and an essentially hydroxyl-free epoxy resin having an epoxide value of from about 0.45 to about 0.75. The composition is essentially free of highly volatile organic solvents. The composition is characterized as having improved storage stability and provides excellent adhesion between elastomeric substrates and flocking fibers, even after exposure to water. The essential adhesive component as claimed by '119 comprises:    (a) an isocyanate terminated polyurethane prepolymer based upon a mixture of difunctional polyols, one having a number average molecular weight of from about 400 up to 1000, and another having a number average molecular weight of from more than 1000 up to about 4000;    (b) aromatic dinitroso compound;    (c) epoxy resin having an epoxide value of from about 0.45 to about 0.75 and a hydroxide value of less than about 0.01; and    (d) at least one C8-C16-alkyl substituted benzene.
In the majority of flock applications where VOC compounds are preferred for rapidly drying of the adhesive coating prior to flocking, a diluent of low volatility cannot be used. In these instances where a VOC is preferred, it would be desirable to reduce the level of VOC emitted from the flocking process. Reducing the level of organic diluent, whether a VOC or less volatile organic solvent, is problematic owing to the effect of the organic diluent on adhesive viscosity. Useful adhesive for coating elastomer substrates must wet the elastomer evenly and not recede or form pools. It would be desirable to provide a solvent-based flocking adhesive which contains reduced VOC and yet provides good wetting and even distribution on elastomeric articles. An adhesive having a Brookfield viscosity at room temperature (˜25-30° C.) of 100 to 1500 centipoise (LV@2/30 rpm) and containing reduced solvent levels would be desirable.
Regulations aimed at reducing solvent emissions and usage of HAP (hazardous air pollutant) or photochemically reactive solvents vary with country, state, and county. Plant location (local regulations), usage, and other factors control what level and type of solvents are tolerable. This invention aims at eliminating HAP and photochemically reactive solvents and further aims at reducing the levels of environmentally preferred solvents. Any improvement in OC levels is desired, <4 pounds/gallon (lbs/gal) but reducing solvent directly compromises viscosity. The present invention enables reduction in solids while maintaining desirable working viscosity at room temperature. Preferred examples show adhesives that target an OC level of 2.5 lbs/gal which is a significant improvement in adhesive VOC's (˜71.5% solids). Such adhesives can be prepared with viscosities in the 125-300 centipoise (cps) range which is preferred (broad window 50-1500 cps). Yet further reduction in solvent is possible, 85-90% solids or beyond—the compromise being to viscosity. Conventional flock adhesives are found typically at less than even 50% solids (4 lbs/gal OC), viscosity at 150-300 (cps), and are in HAP solvents.
U.S. Pat. No. 5,185,402 discloses a flock adhesive based on a combination of two distinct polyurethane prepolymers and a film-forming component. A first polyurethane prepolymer is derived from poly(alkylene oxide) polyols and a second polyurethane prepolymer derived from poly(alkylene oxide) polyols and at least one dimer acid polyol. The preferred polyols suggested for preparation of the prepolymer are a combination of polyols which differ in molecular weight. Specifically, the '402 patent teaches a preferred combination of a poly(alkylene oxide) polyol having a molecular weight in the range of from about 600 to 1,400, preferably about 800 to 1,200, with a second poly(alkylene oxide) polyol having a molecular weight in the range of from about 1,600 to 2,400, preferably about 1,800 to 2,200. However, '402 does not teach the effect of the isocyanate terminated prepolymer upon viscosity in a solvent-containing adhesive.
Experience has shown that prepolymer and level of organic diluent in flock adhesives containing isocyanate-terminated prepolymers significantly affects viscosity and the degree of coatability. Owing to increasing environmental limits being imposed by processors of volatile organic components levels, the amount of solvent should be minimized, however with some prepolymers, the level of solvent necessary to provide a low viscosity for coatability remains a challenge. Improved low viscosity, low VOC, flock adhesives based upon isocyanate terminated polyols which exhibit good fiber and substrate adhesion strength over a wide service temperature range would be highly desirable.
Of primary concern for solvents used in the flock adhesive art, the aromatic solvents often employed in conventional flock adhesive products are cited in the 1990 Clean Air Act as hazardous air pollutants (HAPs), and are targeted for elimination under the National Emission Standards for Hazardous Air Pollutants. Aromatics also have low permissible exposure limits (PELs) and contribute to workplace health hazards. Finally, aromatic solvents have an extremely low level of biodegradability. This causes concern from a waste treatment standpoint or if the product were to migrate into natural waterways. A flock adhesive absent HAP, or a minimized level of HAP would be desirable.